United States patent application US 2008/0129398 describes designs for a digitally controlled oscillator (DCO) for replacing conventional voltage controlled oscillators (VCOs). The DCO comprises an array of switchable capacitive elements, for example varactors, constituting a capacitive arrangement. The capacitive arrangement determines the frequency of the DCO, and has a large number of switchable capacitive elements, each element enabling a frequency step in an HF frequency synthesizer, e.g. having steps of 4 kHz.
A problem of the device known from US 2008/0129398 is that for each step an additional switchable capacitive element is needed, for example 4096 elements for a required modulation range.